1. Field of the Invention
The present invention relates to data transmission control units, and more particularly to control units of the type which function to control data transmission between data processing apparatus and a disk storage device, such as the type generally known as a floppy disk storage device.
2. The Prior Art
Floppy disk storage devices are manufactured by a variety of manufacturers, and differ from each other in a number of respects. One group of such disk storage devices, however, is distinguished by a common style of formatting of stored data, and such formatting lends a certain degree of standardization to the disk storage devices. In a commonly used format, a number of data storage tracks is provided on each disk, typically 75, and each track is divided into a number of sectors, 26 of which are typically available to store data, and each sector is capable of storing a number of eight-bit characters, or "bytes". Control units which have heretofore been used with such disk storage devices have functioned to read an entire sector from the disk directly into a memory or buffer located at a central processing unit, under control of the control processing unit, with the central processing unit transmitting an entire sector to the disk control unit for writing. Such control units are not able to process information except for certain checking and retry routines, and transmission to and from the central processing unit is in complete sectors only. When a reading operation is not completed properly, repeated retries are preformed successively, during successive revolutions of the disk, with no reading of any other data from the disk.
This mode of operation leads to severe limitations in the speed of operations using the disk storage device, and places an upper limit on the number of operations which can be performed during any given interval, using the disk storage device.
Another limitation of most of the heretofore known disk control units is that they are not able to search for and process logical records which are more or less than a single sector in length.
Because of the need to transfer data directly to a memory of the central processing unit, and the many operations, for checking and the like, which must be carried out by the central processing unit in connection with such a transfer, it is usually not possible to finish the processing of any sector of data until after the next sector has begun to pass under the read/write head. And since sectors are typically processed in sequential order, when this occurs it is necessary to wait for nearly an entire revolution of the disk between sectors which are read. This too limits the average operating speed of prior art disk control units.
When a track selection operation is performed, previously known disk control units such as floppy disk units, which use stepping motors to move the head, function to cautiously advance the track selection mechanism in a series of stepwise movements, with the steps widely spaced so that each step can always be completed before the next is commanded. This also represents a limitation on operating speed.
A host of other disadvantages characterize the apparatus of the prior art, such as lack of flexibility in adapting to different storage formats on the disks, and the need for a case or housing separate from the disk storage unit itself. Many of the prior art devices also suffer from system parameters which allow only very narrow time periods during which certain events can be performed.
Accordingly, it is desirable to provide an apparatus and method for increasing the overall efficiency of use of disk storage devices, by avoiding the limitations which have been described.